Just wanted to give some details on what our phone contractor did to help with the VM light issue. First, they set up a phantom extension for the support mailbox. (It's not linked directly to a phone.) They then setup a user with that extension and enabled dynamic extension express. They setup the three extensions that should have a light flash when there's a message left in the support mailbox ad IP phone 1, IP phone 2 and softphone. Voila, it works! Absolutely no idea why this works, it just does. Hopefully this can help some of you who may run into this same situation. Mitel should definitely address this limitation--it's pretty standard to have a mailbox that needs to blink on more than one phone. Having to use a kluge like this is crazy, but at least it can be done.
Thanks to all who commented.